1. Field of the Invention
This invention relates to a driving device for a display panel in which pixel cells acting as pixels are positioned on each display line.
2. Description of the Related Art
Recently much attention has been paid to plasma display panels (hereafter called “PDPs”) as two-dimensional image display panels. In general, the PDP has a plurality of discharge cells arranged in a matrix form. The subfield method is also known as a driving method to cause the PDP to display an image corresponding to an input image signal. In the subfield method, a display period for one field is divided into a plurality of subfields, and each discharge cell is selectively caused to discharge and emit light in each subfield according to the brightness level expressed by the input image signal. By this means, an intermediate brightness is perceived according to the total light emission period within the whole display period of the field concerned.
FIG. 1 of the accompanying drawings shows one example of a light emission driving sequence based on this subfield method, which is disclosed in FIG. 14 of Japanese Patent Kokai (Laid-open Publication) No. 2000-227778).
In the light emission driving sequence shown in FIG. 1 of the accompanying drawings of this application, one field period is divided into 14 subfields, which are subfields SF1 to SF14. All discharge cells of the PDP are initialized to the lit mode (Rc) only in the leading subfield SF1 of the subfields SF1 to SF14. In each of the subfields SF1 to SF14, discharge cells are selectively set to the extinguished mode (unlit mode) (Wc) according to the input image signal, and only those discharge cells which are still in the lit mode are caused to discharge and emit light over the period allocated to the subfield concerned (Ic).
FIG. 2 of the accompanying drawings shows one example of a light emission driving pattern in one field period, in which each discharge cell is driven based on the light emission driving sequence described above and shown in FIG. 1 of the accompanying drawings (see for example FIG. 27 of Japanese Patent Kokai No. 2000-227778).
In the light emission pattern shown in FIG. 2 of the accompanying drawings of the instant application, each discharge cell which is initialized to the lit mode in the leading subfield SF1 is set to the extinguished mode during one of the subfields SF1 to SF14, as indicated by a black circle. Once the discharge cell is set to the extinguished mode, that discharge cell does not return to the lit mode until the one field period finishes. Hence during the period until the extinguished mode is set, the discharge cell continues the discharging and light emission in the subfields, as indicated by the white circles. Here, the total light emission period in one field period is different for each of the 15 light emission patterns shown in FIG. 2, so that 15 intermediate brightnesses are expressed; that is, intermediate brightnesses can be expressed for (N+1) gray scales (where N is the number of subfields).
However, because in this driving method there is a limit to the number of subfields into which one field can be divided, the number of gray scales is inadequate. In order to mitigate the insufficient number of gray scales, multi-grayscale processing, such as error diffusion and dither processing, is applied to the input image signal.
In the error diffusion processing, each pixel of the input image signal is converted for example into 8-bit pixel data, and the upper 6 bits are taken to be display data while the remaining lower 2 bits are regarded as error data. The result of weighted addition of the error data in the pixel data of the surrounding pixels is then reflected in the display data. Through this operation, the brightness of the lower 2 bits of the original pixel is pseudo-represented by the surrounding pixels, and consequently only 6 bits of display data, fewer than the original 8 bits, can represent brightness grayscales equivalently to the 8 bits of pixel data. Then, the 6 bits of error-diffused pixel data obtained by this error diffusion processing are subjected to dither processing. In dither processing, a plurality of neighboring pixels are regarded as one pixel unit, and dither coefficients consisting of different coefficient values are allocated and added to the error-diffused pixel data corresponding to the pixels within one pixel unit respectively. By means of addition of these dither coefficients, when the one pixel unit is viewed, brightness equivalent to 8 bits can be represented using only the upper 4 bits of the dither-added pixel data. Therefore, the upper 4 bits of the dither-added pixel data are extracted and used as multi-grayscale pixel data PDs, so as to allocate these pixel data PDs to the 15 light emission patterns, as shown in FIG. 2, respectively.
However, if dither coefficients are added regularly to pixel data in dither processing, pseudo-patterns not related to the input image signal, i.e., so-called dither patterns, are sometimes perceived. This detracts from the image quality.